But the Trump administration appears to have backed away from that position, saying the question of whether or not Mr Assad stayed in power was no longer a priority for the US.

"You pick and choose your battles and when we're looking at this, it's about changing up priorities and our priority is no longer to sit there and focus on getting Assad out,” said Nikki Haley, the US ambassador to the UN.

View photos

US Ambassador to the United Nations Nikki HaleyCredit: NICHOLAS KAMM/AFP

Her comments echoed those of Rex Tillerson, the US secretary of state, who said Thursday that “the longer-term status of President Assad will be decided by the Syrian people”.

Hours after the new American position was announced, Boris Johnson, the foreign secretary, reiterated that Britain still believed Mr Assad had to go.

“For the long term good of the Syrian people there must be a transition away from the Assad regime, which has dealt so much death and destruction to the people of Syria,” he told a Nato summit in Brussels.

Michael Fallon, the defence secretary, said the UK does not “see a long term future in Syria for someone who has been bombing his own civilians”.

View photos

Boris Johnson has long said since becoming foreign secretary that Mr Assad must goCredit: AFP PHOTO / Daniel LEAL-OLIVAS

The Trump administration’s position drew fire from some Republican senators, who said it was not acceptable for the US “to ignore the wholesale slaughter of the Syrian people by the Assad regime”.

Lindsey Graham, a hawkish Republican senator, warned Mr Trump that allowing Mr Assad to stay in power was “a great reward” for both Russia and Iran, who have strongly backed the Syrian regime in its fight against rebel groups.

The High Negotiations Committee, a Syrian opposition umbrella group, said it was dismayed by the US remarks and that it would “never accept any role” for Mr Assad in Syria’s future.

The significance of the Trump administration’s policy shift remains to be seen.

Mr Trump has said in the past he would be open to cooperating with Russia in Syria in the fight against the Islamic State (Isil) and a toning down of US opposition to Mr Assad could be a step towards that rapprochement.

View photos

Mr Trump has been less critical of Mr Assad than Barack Obama was Credit: AP Photo/Pablo Martinez Monsivais

But with Mr Trump’s White House besieged by allegations about Russia it may be too politically difficult for the president to broker any kind of military cooperation agreement with Moscow.

While the Obama administration remained rhetorically committed to seeing Mr Assad step down, in reality it did little to try to force him from power and officials privately conceded that he was likely to stay.

Under Barack Obama the US gave some arms to Syrian rebel groups but most of its focus was on equipping forces to fight against Isil rather than the Syrian regime. Mr Trump's policy places even heavier emphasis on the anti-Isil front and even less on the opposition to the Syrian regime.

Mr Johnson has also conceded that Britain’s years of opposition to Mr Assad has yielded few results and said in January that the UK was open to seeing Mr Assad stand for election if there as part of a broader peace agreement.

“It is our view that Bashar al-Assad should go, it’s been our longstanding position. But we are open-minded about how that happens and the timescale on which that happens,” Mr Johnson said.

“I have to be realistic about how the landscape has changed, and it may be that we will have to think afresh about how we handle this. The old policy, I am afraid to say, does not command much confidence.”